Recent models show that the best explanation for the CO gas observed co-located with exo-Kuiper belts is a secondary production (i.e., the gas is not a remnant of the young planet-forming disks that persist for less than 10 Myr), where CO is released from planetesimals at a rate proportional to their collisional frequency [1]}, [2]}. Observations of carbon and oxygen atoms are explained as the daughter species of CO photodissociation within the framework of this model. The most massive gas disks with CO masses close to the amount observed in younger planet-forming disks have first been called hybrid disks [3]}. We can now also explain the gas in these previously considered hybrid disks as entirely secondary, because CO released from planetesimals becomes shielded by the carbon produced when it photodissociates (and by CO itself through self-shielding), which can then accumulate to large amounts [4]}, [5]}.
